1. Field of the Invention
The present invention relates to power supplies and high voltage fault protection. More specifically, the present invention relates to high voltage power supplies for free electron lasers and other devices.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Certain devices require a stable high voltage pulse power supply for proper operation. For the purpose of the present invention, a high voltage pulse power supply is a power supply operating at voltages of 100 kilovolts to 1 megavolt at currents in excess of 1 or 2 kiloamperes with microsecond switching times and pulse lengths of tens of microseconds producing constant current or constant voltage waveforms on typical loads.
Typical loads include harmonic amplifiers and free electron lasers (FELs). An FEL, for example, is device in which a beam of electrons is passed through a spatially varying magnetic field. The magnetic field causes electrons in the beam to "wiggle" and radiate energy. In this device, the high voltage power supply performs the function of accelerating electrons to provide the high energy input electron beam. More specifically, FELs and certain other devices require accurate applied voltages on the order of one part in one thousand. Otherwise the frequency and power gain of these devices change with time and produce undesirable results.
Unfortunately, for such applications, it is difficult to sustain the voltage levels for the desired pulse lengths because of the high voltage and current requirements. That is, the high voltage and current levels are difficult to switch and are problematic with respect to arcing and other hazards. For example, operation at high voltage and current levels requires some high energy storage at powers typically in the megawatt range. However, when a megawatt of power is delivered for a second or more, a megajoule of energy is developed. At these energy levels, a failure or an arc may produce an electrical explosion and heat comparable to a pound of explosives. Accordingly, it has been recognized as being desirable to provide power regulation devices and pulse power supplies which operate for short durations with control of pulse length and amplitude while switching on and off at will. An additional class of applications exist within the power utility and power distribution industry. Here better means of controlling high voltage, particularly for interrupting current against high voltages are required.
Current power modulating and regulating technologies include MOSFETS and FETS, GTO/GAT and SCRs, voltage regulator tubes and saturated reactors. Metallic-oxide semiconductor field effect transistors (MOSFETS) and field effect transistors (FETs) are limited in current and voltage handling capability and as with other solidstate devices, these arrays must be carefully snubbed to protect against catastrophic failure. (Snubbing involves the use of auxiliary resistors and capacitors to absorb transients.)
GTO/GAT (gate-turn-off/gate assisted turnoff) devices and SCRs (silicon controlled rectifiers) are limited in speed and voltage and generally have large control power requirements. Voltage regulator tubes are also limited in current handling capability and are unstable in series arrays. Saturated reactors are used to pulse form or sharpen pulses but are not a solution to the general problem of opening switches.
The present high voltage pulse power supply art includes transformers, Marx banks and pulse forming networks. Transformers operating at high voltages have difficulty with respect to the voltage time product. That is, the duration at which a high voltage is sustainable is limited by the nature of the transformer. Increases in sustainable pulse lengths come at a cost in terms of transformer size and weight and in the rise and fall times of the pulse. Output pulse length can not therefore be arbitrarily increased.
Nonetheless, transformers have several features which allow for some degree of tradeoff in design. Typically the tradeoff is between pulse rise time and pulse flatness. Unfortunately, certain devices, such as free-electron lasers, are designed to operate at a specific voltage. When the transformer is initially turned on, before reaching the operating range of the device, the voltage rises through some range in which the device may experience undesirable and perhaps damaging effects. Accordingly, in FEL and other applications, it is not desirable to initiate a pulse with a long rise time as the electron beam will not be properly focused during the rise of the pulse.
A Marx bank is a set of capacitors switched ("erected") from a parallel configuration, in which the capacitors are charged, to a series configuration from which the stored voltage is discharged. Essentially, the erected Marx bank acts as though a simple capacitor is driving the load. While this may be the currently simplest preferred approach, there is a significant problem associated therewith. That is, as the sustainability of the output pulse is defined by the resistive-capacitive (RC) time constant of the device, long steady pulses require long RC time constants. Thus, the extent to which the supply voltage can be sustained at the desired steady state level is dependent on the capacitance of the bank. Unfortunately, as the capacitance of the bank goes up, the hazards associated with the operation of the device go up due to the relationship between the energy stored and the capacitance.
In addition, conventional Marx banks are typically implemented with noninterruptible switches. As a result, as the output current decays exponentially below the specified current range or pulse length the remaining energy stored in the device is essentially wasted. Thus, while slowing the operation, this limitation impacts adversely on the efficiency of the device.
The use of pulse forming networks in lieu of capacitors addresses the shortcoming of conventional Marx banks to some extent. However, elements in the pulse forming networks induce oscillations in the output of the power supply. These oscillations are difficult and expensive to remove and may be unacceptable for certain applications. In addition, Marx banks, composed of pulse forming networks, output at half the voltage level of purely capacitive Marx banks. Therefore, input and stored voltage levels must be twice as high as alternative designs.
Thus, there is a need in the art for an efficient controllable high voltage power supply or regulator/modulator.